Abstracts

RATIONALE: In epilepsy, magnetoencephalography (MEG) is a diagnostic procedure ordinarily employed to localize interictal discharges. Ictal events are generally not recordable using MEG because a) the requirement that the patient remain completely immobile precludes recording durations long enough to capture seizures, and b) movement artifact obscures MEG waveforms. Development of MEG localization tools for epilepsy has been directed towards use on interictal discharges. In the course of MEG recording, however, subclinical seizures are sometimes obtained. We have examined the detection and localization characteristics of MEG and EEG for subclinical seizures. The aim of this study is to evaluate the differences between subclinical ictal patterns seen on MEG and EEG in terms of identification and localization.
METHODS: We studied 43 consecutive epilepsy patients who 1) underwent continuous video-EEG monitoring, 2) had simultaneous EEG and MEG recording for an average of 22.0 minutes, 3) proceeded to epilepsy surgery, and 4) were followed for at least one year. Ictal and interictal discharges were identified and localized from the raw EEG and MEG waveforms independently by separate investigators who were blinded to the clinical information. Detection and localization of subclinical seizures were compared between EEG and MEG.
RESULTS: Subclinical seizures were seen in 4 patients (See Table). The seizure interpretation was concordant between EEG and MEG in one case (Case 8). In the cases where the seizure interpretation was not concordant (Cases 2, 14, 44), the other modality showed simultaneous interictal activity in the same region. In cases 2 and 14, rare spiking was seen during the corresponding ictal pattern. In case 44, the EEG showed runs of spikes lasting for the duration of the MEG seizure pattern. The localization of the seizures was supported by clinical diagnosis in all cases.
CONCLUSIONS: From visual review of the amplitude vs time waveforms, there were differences in seizure identification between EEG and MEG. Some differences may be due to inter-investigator difference in interpretation. There are also theoretical differences in the sensitivity to epileptiform discharges that can contribute to a morphological difference. MEG recording during the presurgical evaluation of epilepsy patients may provide useful ictal as well as interictal information.[table1]
[Supported by: This study was partly supported by Uehara Memorial Foundation.]